The present invention relates to a method for the preparation of a perfluoroalkyl group-containing organopolysiloxane or, more particularly, to a method for the introduction of 3-(perfluoroalkyl)propyl groups into an organopolysiloxane, by which the content of fluorine in the product organopolysiloxane can be freely controlled.
As is well known, organopolysiloxanes having fluorinated hydrocarbon groups, e.g., 3,3,3-trifluoropropyl groups, bonded to the silicon atoms are widely used as the principal constituent of various kinds of silicone products by virtue of their unique properties not obtained in conventional organopolysiloxanes having unsubstituted hydrocarbon groups only as the organic groups.
Although various synthetic methods are possible as a means for introducing such fluorinated hydrocarbon groups into an organopolysiloxane, the only method industrially established heretofore utilizes the so-called hydrosilation reaction between a vinyl group and a hydrogen atom directly bonded to a silicon atom of a silicon compound. For example, a diorganopolysiloxane having a methyl 3,3,3-trifluoropropyl siloxane unit of the unit formula (CH.sub.3)(CF.sub.3 CH.sub.2 CH.sub.2)SiO is prepared by first reacting 3,3,3-trifluoropropene-1 of the formula CF.sub.3 CH.dbd.CH.sub.2 with methyl dichlorosilane of the formula HSi(CH.sub.3)Cl.sub.2 to give methyl 3,3,3-trifluoropropyl dichloro silane of the formula CF.sub.3 CH.sub.2 CH.sub.2 Si(CH.sub.3)Cl.sub.2 which is hydrolyzed to give a cyclic trimer 1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl)cyclotrisiloxane followed by the ring-opening polymerization thereof.
Generally, a 2-(perfluoroalkyl)ethyl group-containing organosilicon compound can be prepared by the hydrosilation reaction of a perfluoroalkyl ethylene of the formula Rf--CH.dbd.CH.sub.2, in which Rf is a perfluoroalkyl group, with an SiH-containing organosilicon compound. For example, U.S. Pat. No. 4,898,958 corresponding to Japanese Patent Publication 3-75558 discloses a reaction of a perfluoroalkyl ethylene with methyl dichlorosilane to give a 2-(perfluoroalkyl)ethyl methyl dichlorosilane which is reacted with 1,3-dihydroxy-1,1,3,3-tetramethyl disiloxane to give a cyclic trisiloxane of 1-(2-perfluoroalkylethyl)-1,3,3,5,5-pentamethyl cyclotrisiloxane followed by a ring-opening polymerization reaction thereof to give a diorganopolysiloxane consisting of the siloxane units of the unit formula --[--(--SiMe.sub.2 --O--).sub.2 --SiMe(CH.sub.2 CH.sub.2 Rf)--O--]--, in which Me is a methyl group and Rf is a perfluoroalkyl group.
While the above mentioned hydrosilation reaction is applicable not only to an organosilane compound having an --SiH group but also to an organopolysiloxane compound provided that the compound has at least one silicon-bonded hydrogen atom, at any rate, the hydrosilation reaction for introducing a fluorinated hydrocarbon group into a silicon compound in the prior art is conducted solely by using the above mentioned perfluoroalkyl ethylene so that the fluorinated hydrocarbon group introduced into a silicon compound is limited to 2-(perfluoroalkyl)ethyl groups. The reaction of this type has a problem or disadvantage that the selectivity of the reaction is so low that only a part of the silicon-bonded hydrogen atoms can pertain to the hydrosilation reaction even by setting aside the problem that the fluorinated hydrocarbon group to be introduced into the silicon compound is limited to 2-(perfluoroalkyl)ethyl groups.